Treating sewage



July 25, 1939.

J. G. BEvAN TREATING SEWAGE Filed Dec.

INVENTOR JOHN Gu B EVAN BY f ATTORNEYS ."rmedlll-YLZS-@iw a 1^2,161,443*

- UNITED sigari-:Sv PATENT OFFICE? TREATING SEWAGE John `G. Bevan, NewYork, N. Y., assignor to Guggenheim Brothers, New York, N. Y., acopartnership Application December 28, 1934, Serial No. v'159,450

5 Claims. (Cl. 210-2) This invention relates to sewage treatment and Thepresent invention contemplates certain has -for an object the provisionof an improved improvements in the removal of putrescible mat' processfor treating sewage and similar waste' ter'from sewage by coagulation,ilocculation or liquors. More particularly, the invention conentrainmentin a settleable sludge, and the pro- 5 templates the provisionof animproved process for vision of a flexible process capable of being ad- 5removing putrescible matter from sewage and justed readily to treateffectively sewages in similar waste liquors. The invention furtherwhich the amounts of putrescible matter are subcontemplates theprovision of an improved exiject to wide fluctuations. The inventionfurther ble process for treating sewage in which the contemplates theprovision of an improved procl amounts of putrescible matter are subjectto varess, involving removal of suspended kputrescible 10 iation. Afurther object of the invention is to matter, in which removal ofdissolved putresprovidev an improved two-stage process for reciblematter is also effected. I have discovered moving putrescible matterfrom sewage and simthat the removal of putrescible matter (susilar wasteliquors. pended and dissolved) from sewage is promoted i `According to apreferred two-stage process of by subjecting the sewage to a preliminaryaera- 15` vthe invention, raw sewage, after screening, is tion treatmentin the presence of returned sludge l subjected lto controlled aerationto effect coagucontaining putrescible matter coagulated during lation ofa portion ofthe putrescible vmatter in the course of the aerationtreatment, and subthe absence of any substantial amount of added jectingthe residual sewage, after separation of sewage, -after separation ofthe coagulated pucoagulated during the aeration treatment, to tresciblematter is subjected to theaction of a the action of a coagulating agent.'I'he treatchemical coagulating agent to eect coagulation ment of the'residual sewage is preferably conof a further portion of the putresciblematter. ducted in the presence of returned sludge con- Preferably, theaeration treatment of the'raw taining putrescible. mattercoagulatedduring the 25 -sewage is conducted in the presence of returnedcourse of the treatment of residual sewage with sludge containingputrescible matter coagulated a coagulating agentduring the aerationtreatment of raw sewage and The aeration SBD'OI' Stage 0f the completeDIOC- without the addition of any chemical coagulating ess of theinvention is designed to effect coaguagent. The treatment of theresidual sewage is lation of a portion only of the suspended and dis- 30conducted in the presence of returned sludge solved putrescible matter.Coagulation of an containing putrescible matter coagulated duringadditional amount of putrescible matter suillcient the treatment Iofresidual sewage with a coagu- .l to lpermitA the production of asuitably pure or latins agent. clean emuent liquor is eifected in thestage or 35 In ordinary raw sewages and similar waste step of theprocess in which an added coagulatliquors, putrescible matter,consisting of carbon, ing agent is employed. When the amount ofsushydrogen, sulphur, etc. in various combinations, pended matter in thesewage is below average or iS present in WU fOrmS, namely: (1) insolublenormal, the aeration treatment effects condition- (suspended matter),(2). soluble. In any sewing of the putrescible matter to such a degree40 age treatingoperation, it is customary to remove that very smallamounts of coagulating agents 40 the coarser' insoluble matter byscreening. After are required in the second stage or step. Thescreening, the concentration of the insoluble pusecond stage or step ofthe process of the inventrescible matter remaining in the sewage isextion may be carried out with the use of any tremely low, for example,an average municipal desirable amount of coagulating agent to prosewagecontains around 1 50 to 200 parts per milvide protection againstunpredictable and unex- 45 lion of dispersed or finely divided solidmatter pected changes in the character of sewage. In largely incolloidal suspension. This finely diany event, the adjustment of theamount of vided putrescible matter includes simple and coagulatingagentemployed maybe accomplished complex compounds resulting. from thebreaking simply to give whatever rapid and effective condown ofproteins. The amount of putrescible trol that may be required for thetreatment of 50 matter in the sewage ,froml any locality varies atsewages varying in strength.

different periods-of the`day and night, and it is The process of theinvention has many advandeslrable to employ' for the treatment of sewagetages in addition to those mentioned above. Fola' process which can bereadily adjusted in aclowing are some of the additional advantages:

cordance with such variations., (1') A considerable portionof theammonia is 55 chemical coagulating agent, and .the residual the returnedsludge and the putrescible matter 20l destroyed during the' course ofthe aeration treatment. Experimental results indicate that as much ashalf of the ammonia is oxidized.

(2) A part of the putrescible matter in the sewage is coagulated duringtheaeration treatment. The coagulating agent employed for treatment ofthe residual sewage completes the desired coagulation and permits theproduction of a very clear eilluent having a low B. O. D. (5 day). Thereturned sludge containing putrescible matter coagulated during thetreatment of the residual sewage with the coagulating agent isespecially eiective as an aid in clearing the eluent of suspendedmatter. I

(3) Certain of the compounds in solution in the sewage are partiallyoxidized during the aeration treatment, and the partially oxidizedcompounds are precipitated when the residual sewage is subjected to theaction of the coagulating agent.

(4) The organic nitrogen-remaining in the residual sewage is largelydestroyed in the stage o r step ofthe process in which the residualsewage is subjected to the action of an added coagulating agent. This isespecially true when the residual sewage is subjected to the action ofsludge containing putrescible matter coagulated during the treatment ofthe residual sewagewith an added coagulating agent, 30*

(5) Theprocess of the invention permits economy in the'use ofcoagulating agents. Effective elimination of putrescible matter can beaccomplished through the use of relatively small amounts of chemicalcoagulating agents.

Satisfactory aeration is obtained with from about 0.01 to 0.1 cubic feetof air per gallon of sewage treated. The period of aeration may bevaried considerably. Good results may be obtained if aeration in thefirst stage or step of Any suitablech'emical coagulating agent may beemployed in treating the residual sewage, and any suitable method oftreating the sewage with the coagulating agent may be employed. I mayemploy,for example, aluminum chloride', aluminum sulphate; ferroussulphate; ferri'c sulphate;

ferric chloride; ferrous oxide; ferric oxide; bauxite;` ferriehydroxide; chlorine; chlorinated copperas; sulphuricracid; manganesesulphate; Vacid. treated sludge, such for-example, as sludge produced inthe treatment of sewage with an iron compound, after treatment withsulphuric acid in amount suicient'to react with the iron containedtherein to produce ferrie' sulphate; incinerator ash produced inincinerating sludge produced in 4treating sewage with a coagulating` maybe ad-ded to the `residual sewage separately or in combination at anysuitable time or point, and the-y may be added in the solid condition orl in solution in suitable solvents.

For the second stage or step of the process of vessel.

No. 1,956,420 dated April 24, 1934; 'or the process described in mycopending application Serial No. 738,781, 'led` August 7, 1931i (PatentNo. 2,021,679, dated November 19, 1935).v i

In carrying 'out the first stage or step of a preferred process of myinvention, the returned sludge is preferably added to the sewage at thecommencement of the aerating treatment or shortly after the raw sewageenters the aeration Following the aeration treatment, the sewage isintroduced into a clarier underneath a layer or blanket of settlingsludge so that the In carrying out the second stage or step of theprocess of my invention, I may disperse the coagulating agent in theresidual sewage in any suitable manner; as, for example, by-mechanicalagitation or by air agitation. Air agitation may be carried out in anysuitable manner as, for example, by injecting one or more streams of airinto the body of liquid to be aerated or by employing mechanicalagitation means capable of effecting the introductionvand dispersion ofair. Air agitation and mechanical agitation may be employed together inany suitable manner. I prefer to employ air agitation, in accordancewith the procedure outlined in my aforementioned copending application,to obtain the combined advantages of dispersion of the coagulat- ,ingagent and oxidation of impurities by the oxygen of the air.' For the`treatment of weakduces satisfactory results. Effective. coagulation ofthe putrescible matter of the 'residual sewage generally canbeaccomplished by vtreatment of the residual sewage with a coagulatingagent for a period of about 5 to 30 minutes.

In vcarrying out the second stage or step of a preferred. process of theinvention, the returned sludge is added to the residual sewage at thecommencement 'of the aerating treatment or -shortly after the residualsewage enters the aeration vessel. A coagulating agent, such, forexample, as a metal compound may be added -to the residual sewage at anydesired stage of the aerating treatment. When a coagulating `agent suchas ferric sulphate is employed, it is preferably added to the residualsewage during the course of its aeration within about two to fifteenlminutes prior .to' the completion of the aerating treatment. Aluminumsulphate also is preferably added within about two to fifteen minutesprior to the completion of the aeration treatment. Ferrous sulphate ispreferably added to the sewage within about ten to thirty minutes beforethe end of the aeration treatment.

Good results may be obtained if the period of aeration following .theaddition of theferric sulphate or' aluminum sulphate to the residualsewage Ais just suicie'nt to permit thorough dissemination of thecompound. The period of aeration may be varied considerably. Goodresults may be obtained if aeration of the residual sewage is carriedout for a period of about one-half hour or less, and little vor noadditional advantage may lresidual sewages, mechanical agitation aloneprobe gained by continuing aeration fora period of time longer thanabout one hour'. y l

After theiirst aeration stage, if the pH of the residual sewage is 7 orlower and 'it is desired .to increase the pH to'a value abo re 7, anamount of .an alk'aline (gent such as lime suicie'nt to increase the pHto the desired'value may be added 'prior tothe completion of thetreatment with. the coagulating agent. Usually, it will not beneces-V'sary/or desirable. to adjust the pH of the residual.

sewage by-the addition of an alkaline agent, 'as I have found thathighly satisfactoryprecipitation, settling and clariiication can beobtained even when the pHSis considerably below 7.' If the ad dition. ofan alkaline agent is desired or required.

^ for any purpose, it is preferably added'after the coagulating agenthas'4 been added and thoroughly .advantageously by the'addition of thealkaline agent to the nal or'secondary clariiier eiliuent'."Adjustmentof the pH at this point requires the use of -smaller amountsof alkaline agent, two to ilve pants per; million 'usually beingsufiicient. When 4ferrous sulphate is employed as lthe coagulatingagent, it is advisable to adjust the pHby the addition of an alkalineagent such, for example, as lime'. A pH of about '1 may be satisfactory,but a pH of about 7.6 to 7 .8 is preferred.l

Following the .treatment with a coagulating agent, the sewage isintroduced into a clariiier underneath a layer'orv blanket of 'sett1ingsludge so -that the treated` sewage flows upwardly throughfthe sludgeblanket and overfiowsfrom the clarier with a-practically negligibleamount of suspended matter.

In carrying out the process of the invention in its preferred'and'complete form, the raw sewage after appropriate screening is mixed withre-` turned sludge in a primary aeration vessel and aeration of themixture is commenced. .The primary aeration'vessel employed ispreferably of the type whichv permits a continuous] flow of th'e sewageundergoing treatment from an entrance end to-'a discharge end and therate of ilow is preferably controlled to provide a period of about onehour between the time of entry and tlie time of discharge. of thesewage. Aeration' of the sewage is preferably continued throughout itsentire period'of detention 'in the primary aeration 'vessels' The-fisststage or step of the process of the invention is preferably socontrolled that elimination of an appreciable' amount of the.putrescible matter of the sewage undergoing treatment is effected. Thefirst stage or step offment the invention may be controlledadvantageously to eiect elimination of about 50% r more of theputrescible matter o'f the raw sewage.

Following the aeration treatment in the primary aeration vessel, thesewage passes to a primary clarifler, into which it is .introduced nearthe bottom, passing upwardly through a blanket `of settling sludge.,Residualsewage overows and settled sludge is removed from the bottom ofthe-primary clarifier, preferably. being. raked or otherwisemechanically-worked along the bottom of the clarler'- toward the sludge'fdischargeoutlet.

The sludgenom th bottom of the primary elariiier may be conducted inpart to a primary"` thickener' and in part to the primary aerationvessel. Residual sewage overilows and settled sludge is removed from thebottom of the primary thickener. The primary `(':lariiler 'and primarythickener eiiluents may be `combined for subsequent treatment, 'or theprimary lthiclrener emuent may' be returned to the yprimaryaeratlonvesse1for further treatment. The sludge from the.. l

primary thickener, containing about 3% to' l'0% solids, may be.conducted in part to the primary' aeration vessel and in part to aprimary suction filter where its moisture content is reduced. All of thesludge` returned may be obtained from the primary clarifier, all .may beobtained from the .primary thickener,or a portion may be obtained from#the primary thickener and a portion from the^primary clarier. Aboutt`o-75%, more or less, of the sludge is returned to the primary aerationvessel. The amount of 'sludge returned y may vary considerably..Ordinarily, sludge re.

turn maybe controlled advantageously to estab- -lish lin thesewage-sludge mixture undergoing treatment vin 'the primary aerationvessel an amount of suspended matter equal to about two to eight timesthe amount of suspended matter in the incoming raw sewage.

'.Ihe residual sewage from the primary clarler,

or the residual sewage from the primary clarier and the primarythickener in the event that the primary clarifier and primary thickenereilluents are combined, 'is mi'xed with returned lsludge in a secondaryaeration vessel and Jaeraticin of the mixture is commenced. Thesecondary aeration vessel advantageously may be of the same 'typeaeration vessel. Themate of` flow of the residual sewage through thesecondary aeration vessel is preferably controlled to Iprovidea periodo! about 30 minutes between the time of entry and'- the time 'ofdischarge of the `residual sewage.l

Aeration of the vresidual sewage. is preferably continued throughout itsentire period of detention in the secondary aeration vessel.

As the mixture undergoing treatment is flowing through the secondaryaeration vessel, a coagulating' agent' such as a soluble iron compound,

preferably in the 'form of a ferric sul/phate solu. tion of 5.0 gramsper liter concentration isadded y tothe mixture at a point correspondingto an interval of about 2 to 15 minutes before the time o! discharge ofthe mixture from the vessel. Prompt diffusion of the solution throughoutthe from aeration. The oxygenin the air keeps lthe iron in the ferriccondition. When a ferrous salt lsuch as`fe'rrous'sulphate is employed,the oxygen Aof the air converts the iron to the ferric condition.

IThe incomingraw sewage ordinarily has a pH of around 7 to 7.5. In thecoagulating'treatof the residualsewage in the secondary' aerationvessel, the trivaleit metal ions-are precipitated, probably in the form.of ferric hydroxide as Well as inthe form of simple and complex organicferric compounds. The sul phate ions of the added ferric sulphate tendto lower the pH ofthe sewage to a value below '1,

mixture is produced by the .agitation resulting but this tendency islargely cou'nteraoted by the effects of the aeration and thereturnedsludge;

4and. theresidual sewage at the conclusionof the ferric 'sulphatetreatment may have a pH equiva- -lent to that of the incoming rawsewage.Substantially all the. iron added as ferric. sulphate Iii:

which acts as a collector of suspended matter. 'Ihe returned sludge andthe Aaeration treatment also aid in effecting coagulation of thesuspended matter. considerable amountof the dissolved putresciblematter. Y

Tn'e'amount of iron added to the sewage depends .to some extent atleast, upon the amount of putrescible matter in the sewage. In thetreatment of -the average municipal sewages, 5 to 25 parts per millionof iron products satisfactory coagulation ano precipitation, and,generally, less than 20 parts per million of iron produce satisfactoryresults. In the treatment of indus'- trial waste liquors such, forexample, as dairy plant and packing plant waste liquors, 20 to 50 partsper million of iron may be required toj produce satisfactory coagulationand precipitation. Satisfactory aeration is obtained with a total offrom 0.01 to 0.1 cubic foot of air per gallon of sewage treatedin bothstages or steps of the process. It appears that the oxygen of the airemployed for aeration in the secondary aeration vessel'functions chieflyto oxidize putrescible matter and is not absorbed by the sludge. This isindicated by rapd elimination of dissolved putrescible matter and thepresence of dissolvedoxygen in'the secondary clarifier overflow.

As hereinbefore pointed out, adjustment of the pH of the sewageundergoing treatment in the secondary/aeration -vessel may be dispensedwith, and adjustment of the pH may be carried out by the addition of asuitable agent to the final or secondary clarifier eilluent.

if adjustment of the pH is desirable following the treatment with ferriesulphate, the sewage may be treated in the presence of fan lalkalineagent, such as lime, in amount sufcient to give a pH of the desiredvalue. Adjustment `of the pH toa value above 7 may sometimes improvesettling and filtration. Lime is preferable o other alkaline agents forthis operation because it gives a denser loc and one which settles veryrapidly, and because it isinexpensive. Lime is preferably added to thesewage as milk of lime in 50 grams per liter concentration. Where dis,-solved putrescible .matter is to ,be subsequently removed from thesewage liquor by a base exchange (zeolite) treatment, for example, inaccordance with the process of the aforementioned patent to Gleason andLoonam No. 1,886,267,11,"

towards the sludge discharge outlet. The super-y natant liquoroverflowing from the secondary clarifier is substantially free fromsuspended matter. If exceptionally high purification is desired,including high ammonia removal, theliquo may advantageously be subjectedto a base exchange treatment as described in the aforementioned patentto Gleason and Loonam.

The sludge from the bottom of the secondary clarifier may be conducted/in part to a secondary l thickener and in part to the secondary aerationvessel. Clarified liquor overflows .and settled sludge is removed fromthe bottom of the secon- Also, aeration causes oxidation of a darythickener. The secondary clarier and secondary thickener eilluents maybe/combined for appropriate disposal or for subsequent treatment,

ybut the secondary thickener efiiuent is preferably returned to theecondary aeration vessel for retreatment with he residual sewage. Thesludge from the secon ryithickener, containing about 3%- to 10%' solids,may beconducted in part to the secondaryv aeration vessel and in part toa suction -filter where its moisture content is reduced. For filtering,the excess sludge from the secondary thickener may be combined with theexcess sludge from the primary thickener or the sludge from the primarythickener and the sludge from the secondary thickener may be treated inseparate filter units. If the sludge from" the primary thickener iscombined with the sludge from the secondary'thickener for filtration;the filter efiluent or ltrate is preferably returned t the secondaryaeration vessel or to the secondary thickener. If the sludge from theprimary thickener and the sludge from the secondary thickene arefiltered separately, the filter efiluent resulting from filtration ofthe sludge from the primary thickener is preferably returned; to theprimary thickener or'to the primary aeration vessel, and thefiltereiiluent resulting from filtration of the sludge from thesecondary thickener is preferably returned to the secondary thickener orto the secondary aeration vessel.

All of the sludge returned to the secondary `may vary considerably.Ordinarily, sludge re-Y turn may be controlled advantageously toestablish in the sewage-sludge mixture ,undergoing treatment in thesecondary aeration vessel an amount of suspended matter equal to abouttwo to eight times the amount ofsuspended matter in the raw sewage.

Thesludge from the prirnary clarier and the 4 sludge from the secondaryclarifier may be thickened in separate thickeners, as indicated above,

Lor combined and thickened in the same apparatus. When the sludge fromthe primary clarifier andv the sludge from the secondary clarifier arecombined for thickening, the total amount of sludge 'returned to theprimary aeration vessel or basinv is preferablytaken from the primaryclarifier, and the `total amount of sludge returned to the secondaryaeration vessel or basin is preferably taken-from the secondaryclarifier.

' Combining of the sludges for thickening and/or filtration mayeliminate the necessity of conditioning the sludge prior to'filtratiomor may reduce the amounts of reagents required to efectconditioning.v Separate filtration of separately thickened sludges mayrequire reduction of. the

dit

pH of the sludge produced in the first .stage or step by the addition ofan agent such as ferrie sulphate and raising of th pH of the sludgeproduced in the second stage or step by the addition of an agent such aslime or sodium nitrate.

The sludge from thefirst stage ror step of the process of the inventionis suitable for digestion',

and it may be sent to ldigestion instead of being filtered andincinerated. v

It is my preferred practice to completely destroy the organic matter inthe filter cake by injrs ` the entrance end of the primary aerationvessel.

of two sections, namely. ya drying section 'and a burning section isadmirably adapted for the pur' pose. In both the dryer andincineratorsections.

'about G50-700 C. This incinerating temperature is suiilciently high todestroy the putrescible matter. 'The incinerating temperature ispreferably kept below 700 C. so that sintering of the residue isavoided. A substantial amountof the heat re.

" quired for incinerating is obtained by the burningr ciV the sludge. Infact, the caloriiic value of the.

l sludge is suchRthat in a large scale operation no extraneous fuel isrequired for incineration. Y

I'he incinerated residue may be treated .with sulphuric acid for theregeneration of the ferrie sulphate which may be again -used inthesprocess, or theincinerated residue may be' employed as a coagulatingagent in thevsecond stage. of the r'In the apparatus illustrated inprocess of theinvention.` i l The single figure of theaccompanyingdrawing is a diagrammatic owsheet of a system adapted for carrying'outthe process of the invention.

The raw sewage is iirst passed through acoarse screen and then through ane screenA (not shown in the drawing) to remove coarse andheavy solidmatter. The screened sewage is treated'according Ktothe process of theinvention. A

the drawing, aeration'of/the r'awsewage iscarried out in aprimaryaerationvessel in the form of "a long tank or; basin oneend'portion of which receives the incoming sewage and the other endportion of which discharges into a primary clarifier. Returned -sludgeis added to the raw sewage 'asf it enters the primaryaeration basinandthorough of the sludge and sewage is accomplished by air agitatiom Inthis apparatus, the rate of flow of the sewage through the primaryaeration basin is' such as `to give a detention period -about one hour.'A common air diffuser extends throughout the length of the 'primaryaeration vessel and provides a source of air for aeration andagitation.A The air diifuser maybe of any suitable construction. v p lrIhe primary clarifier illustratedin the drawing comprises a tank.having a peripheral charging well 5 `(extendingapproximately around onethird of the circumference ,of the tank) and discharging into the bottomof the tank. I Residual sewage overflows into valaunder 6 opposite thecharging well a'nd extending approximately around onethird' of the.upper circumference of`the tank'. A raking mechanism .1 moves sludgesettling on the bottom of the .tank `vtowards the centrally posi'-tionedsludge discharge outlet. I

Sludge from the primary clarier is pumped by nfeans of a pump Ill to aprimary thickener hav- I ving a sludge raking mechanism Il anda'n.overow launder 9, or in part to the primary thickener and.

' in part to the entrance end'of the primary aeration vessel. Thesludgepump 'I0 communicates with a conduit Il which leads'to a sludgereturn lconduit I2.; A by-pass conduit I3 is provided for conductingsludge from the pump I and 'colfduit Il to theprimary thickener.lSuitablevalves are provided in` the conduits I I and I3 to control anddistribute'thfe ow of sludge-therethrough. The primary clarifier andprimary thickener eiliuents i may be 'combined for. vappropriatesubsequent treatment in the secondary aeration vessel, or thevprimary]thikener eiuent may be returned toJ Conduits I 'I and I8 havingsuitable valves and pumps therein are provided to permitcthe primarythickener eflluent to be combined with the primary .clarier eiiluent fortreatment in the .secondary Jaeration vessel or turned to the entranceend of the primary aeratn vessel. A portion of the ludge from the ener`may be conducted 'by means of a pump Il andconduit I5 to the sludgereturn conduit I2 leading to the entrance end of the primary aerationvessel. 'I'he remainder ofthe-sludge from the primary thickeer'v isconducted through the conduit IB to a primary suction filter.` Suitablevalves are provided in the conduits I5 and I6 to control and distributethe iiow of sludgetherethrough. A conduit I9 communicates with-'theprimary filter and the return conduit I8 and permits the return of theiiltrate to the entrance -end of the primary aeration vessel.

Thesludge returned to the primary aeration vessel to 75%, more or less)may be returned entirely. either from the prim. .2y clarifier or fromthe primary thickener,` or a portion may be returned lfrcm the primaryclarier and a, portion from the primary thickener. The sludge returnedto the primary aeration vessel may be aerated prior to its return. y

Aeration of the residual sewage is carried-out ina secondary aerationvessel milartto the primary aeration vessel and com ising a long 'tankor basin one end portion of which ,receives the incoming residual sewageand theother end portion of 'which discharges into'a secondary clari-Iier. The secondary aeration basin or vessel may' have a capacitysmaller than the capacity of the primary aeration basin or vessel. Therelaq tivesizes ofthe .aeration basins or vessels may be proportional tothe times or. periods of aeration Aemployed in the .two stages or stepsof the process.

The secondary aeration basin or vessel advan` tageously may comprise twoor more 'separately operable compartments to permit variations in* timesor periods of treatment. Returned sludge is added to the residualsewageas it enters .the secondary aeration basinQ' andl 'thorough mixingof the sludge and sewage is accomplished by air agitation. Inthis'apparatus, the rate of ow of y the residual sewagethrough theaeration basin is such'as to give a .detention period of about onehalfhour: 'Ferrie sulphate solution is preferably added to the residualsewage in thel secondary `7aeration basin at a point corresponding to aninterval of about'2 to 15. minutes before the time of discharge of thesewage intothe secondaryk clarifier, but provision is made for theaddition of the ferrie sulphate yat avnumberA of points between theentrancelend land the discharge end of the basin. The ferrie sulphatemay be added. to the residual sewagec in the secondary aeration vessel'at one o r more point's'during the course oi' flow of the sewagethrough the aeration vessel.v

Ifv the .additionl of anw alkaline agent is required or desired foradjustmentof the pH of the sewage, milk of lime is added between theppint of addition of the ferrie sulphate and the discharge of thelferrie sulphate in .the sewage befre'the.

lime is added. IA common air diffuser extends throughout the length ofthe aeration vessel and I provides a source of air for aeration andagitation. The air diffuser 'may be of any suitable construction. l f

'Ihe secondary clarifier is similar to the primary clarifier andcomprises a tank having a peripheral charging well 20 (extendingapproximately around one 'third @i the circumference of the tank) anddischarging into the bottom of the tank. Clariiled liquor overows in alaunder 2i [opposite the charging well and extending approximatelyaround one-third of the upper circumference of the tank. A rakingmechanism 22 moves sludge settling on the bottom of the tank towards thecentrally positioned sludge discharge outlet. It `is to be understoodthat forms and types of clariflers other than those shown and describedmay be used in practicing the invention. I have,found that theintroduction of the sewage near the bottom of a settling chamber4beneath a settling mass of sludge therein materially aids in theproduction of a supernatant liquor substantially free from coagulatedsuspended matter.

I Sludge from the secondary clarier'is pumped by means of a pump 25 to asecondary thickener having a sludge raking mechanism 23 and an overowlaunder 2li, or in part to the secondary thickener andin part to theentrance end of the secondary aeration vessel. The sludge pump 25communicates with a conduit 2d which leads to a sludge return conduitiii.' A by-pass conduit 28 is provided'for'conducting sludge from thepump 25 andconduit 26 to the secondary thickener. Suitablevalves areprovided in the conduits 26 and 28 to control and distribute the ow ofsludge therethrough. The secondary clarifier and secondary. thickenereffluents may be combined for appropriate disposal or for subsequenttreatment, as, for example, by zeolite lters as described in theaforementioned patent to Gleason and Loonam, or the secondary thlckenereffluent may be returned to the entrance end of the secondary aerationbasin. A conduit 32 having a suitable pump and valve included therein isprovided to permit return. of the secondary thickener emuent to theentrance end of the secondary aeration basin;

A portion of the sludge from 'thesecondary' and 3i to control anddistribute the :dow of sludge therethrough. conduit 33 having a suitablevalve -included t ereinA communicates with the secondary ilter and thereturn conduit 32 and permits thev return of the illtrate to theentrance end of the secondary aeration basin.

aromas The sludge returnedv to the secondary aeration vessel (20% to75%, more or less) may be returned entirely either froml the secondaryclarier or from the secondary thickener, or a portion may be returnedfrom the secondary clarifier and a portion from the secondary thickener.

The Vilter cake is conveyed to the dryer and incinerator. Both the dryerand incinerator are rotatably mounted cylinders in which the solidmaterial and gases (for drying and incinerating) travel concurrently.Thus, the exhaust gas from the'incinerator moves through the dryerconcurrently with the sludge, and the dried sludge enters theineineratoralong with hot gases from an oil burner or other heating means.` Anadequate amount of oxygen or.other combustion supporting gas enters theincinerator (along with vthe hot gases of combustion of the extraneousfuel) to burn the organic and other combustible matter in the driedsludge. The -incinerated residueor ash is discharged from theincinerator toa storage container from whence it is appropriately fed toIt will of course be understood that the apparatus illustrated in thedrawing is merely illustrative and is not intended to be restrictive ofthe inventioinv in any respect. I In the following table are given thedaily result obtained in tests conducted for the purpose of illustratingthe results obtainable by means of the process of the present invention.In carrying out the tests of the process of the invention, about of thesludge obtained in the clariiiers (containing about 1% to 4% solids) wasreturned directly from the primary and secondary clariers to theprimary/and secondary aeration vessels, respectively. Aeration wascontinued for one hourperiods in the primary aeration vessel and `twentyminute periods in the secondaryaeration vessel, and ferrie sulphate andaluminum sulphate Were added to the secondary aeration vessel about veminutes before the end of the aeration period.

The biochemical oxygen demand (B. O. D.), suspended matter, iron andaluminum are given in parts per million. The purified eiiluent was thesupernatant liquor from the secondary clarier:

Raw Eiiuent hegls Percent v com DE binea B. o.D eusp.Y 13.0.1) susp.Fe+++ .u Mum@ Nov. 112 s 1o 5 1 c4 1a) 1s 1a 1o 1 e2 17s 11 1s 1s 1 ai24o zo au 5 1 s1 a1 172 12 zo s 1 as Dee. a 18o 19 22 s .s sa 15 25 5vl. 87 11s 1s 2t 7 .s se an 9 a v 1 .s as m 9 u 1o 1 at 172 l 1an 1s als.as 9o duced in processes in common use. It is not un priate time maycauses. better B. v0. D. reduc- Y .tion than can be accomplished inusing ferrie sulphate. For this reason, and because ferrous sulphateisless expensive than ferric sulphate, the aisee! ferrous sulphate may bepreferable. Also, results similar to those obtained in using ferriesulphate may be obtained by employing aluminum sulphate. Mixtures rofvarious compounds may be employedaadvantageously, and desirablev.results ymay' be obtainedfby adding two or more compounds separately.For example, -I have obtained\- extremely good results by adding smallamounts of aluminum sulphate within a minute or two after the naladdition `of ferrie sulphate. The vamountsv of aluminum sulphateemployed were equal to about ten percent of the amounts of ferricsulphateemployed. I have also obtained very desirable resultsbyaddingl asolution made by dissolving Vboth aluminum sulphate and ferrie sulphate,about 1 part of aluminum to 10 parts of iron.

'The process of my .invention is admirably adapted for the treatment ofindustrial waste liquors aswell as ordinary municipal or domesticsewages. Itis characteristic of the process of the invention that anextremely rapidly settling sludgeV is produced .and vhigh elimination ofputrescible matter is accomplished. Sludge produced in carrying out themethod of the invention settles several timesas fast' as sludge prousualto obtain upon standing for ufive minutes aclear supernatant liquidwhich may be deeanted, and I have accomplished B. O. D. reductions ofmore than 96% in the normal operation of the process on many occasions.Preliminary clarication in the primary clarifierusually requires abouttwenty to thirty minutes, and clariiication in the secondary clarifierusually requires about one and one-half to two hours.

I claim:

1. The improvement in the treatment of sewagel containing putresciblematterin suspension and in solution which comprises aerating the sewage.

Vto eifect `coagulation of a portion of the suspended matter and -toconvert a portion of the soluble' matter vto precipitable form,separating the coagulated matter from the residual sewage, re-`coagulated matterfrom the accompanying liquor, v

and returning a sludge containing said coagulated matter to the residualsewage undergoing treatment. f

2. .The improvement' in the treatment of sewage containing putresciblematter whichcomprises aerating the sewage to eiect coagulation of aportion of the putrescible matter, separating the coagulated putresciblematter from the residual sewage, returning at least a part of saidseparated coagulated putrescible matter to the sewage undergoingtreatment by aeration, aerating the residual sewage in the presence ofan added compound of a trivalent metal to eiect coagulation of residualputrescible matter, separating the accompanying liquor, and returning atleast a p art of said separated residual putrescible matter to theresidual sewage undergoing treatment.

3. The improvement in the treatment of sewage containing putresciblematter which comprises aerating the sewage to effect coagulation of aportion of the putrescible matter, separating the coagulated putresciblematter from the residual ,sewage, 'returning at least a part of saidseparated coagulated putrescible matter to the sewage undergoingtreatment by aeration, subjecting the residual sewage to the action of asoluble compound of a trivalent metal to eiect coagulation of residualputrescible matter therein, separating thecoagulated residualputrescible matter from the accompanying liquor, and returning at leasta part of said separated residual putrescible. matter to the residualsewage undergoing treatment.

4. The improvement in the treatment of sew-` age containing putresciblematter which comprisesy aerating the sewage to eiect coagulation of aportion of the lputrescible matter, separating the coagulatedputrescible matter from the residual'sewage, returning a sludgecontaining `the -separated putrescible matter to the sewage undergoing.treatment by aeration, aerating the residual sewage in the presence ofadded ferric sulphate to effect .coagulation of residual putresciblematter, separating the coagulated residual putrescible matter fromthe'accompanying liquor, and reurning a sludge containing the separatedresidual putrescible matter to the residual sew age undergoingtreatment.

5. The improvement in the treatment of sewage containing putresciblematter which comprises aerating the sewage to eiect coagulation lof aportion of theputreseible matter, separating the coagulated putresciblmatter from the residual sewage, returning a least a part of saidseparated coagulated putrescible matter to the sewage undergoingtreatmentby aeration, subjecting the residual sewage to the action ofsoluble compounds of aluminum and iron to eiiect coagulation of residualputrescible matter there-k in, separating the coagulated residualputrescible matter from the accompanying liquor, and res turning atleast a part `of said separated residual putrescible matter to theresidual sewage undergoing treatment.

JOHN G. BEVAN.

` the coagulated residual putrescible matter from l

